In the production of plastic parts by the reaction casting process, the reaction mixture is combined with fillers like, for example, glass fibers or carbon fibers to improve the mechanical properties of the finished parts. For this purpose, one or both of the chemically reactive components of the plastic have fibers mixed therewith. However, only very short fibers (about 0.2 mm in length) can thus be incorporated since otherwise the viscosity of the reactive plastic component will be increased to the point that, with high pressure injection, there is not sufficient admixing of the components. Furthermore, when long fibers are used in a high pressure injection of the plastic components into the mixing chamber, fiber breakage occurs.
From DE-OS 28 23 189 it is already known to feed glass fiber into the mixing chamber of a polyurethane apparatus via a supply passage, together with a portion of one of the components of the chemically reactive plastic. Since this component of the material charged with glass fibers can only be fed to the mixing chamber in a pumping process with the pressure prevailing in the mixing chamber, only short fibers can be used if breakage of the fibers during the pumping process and upon injection into the mixing chamber is to be avoided. The material stream fed additionally into the mixing chamber via the feed passage also creates problems with the maintenance of precise stoichiometric proportions which must be insured in the mixture of the chemically reactive plastic components so that there is no guarantee of a constant quality of the end products.
From German Patent Document 44 17 596 it is known, further, to introduce via a device of the aforedescribed type, the filler via a separate passage into the calming chamber by means of a fluidizing gas. The introduction of the filler is thus effected, however, only at one location along the edge region of the flow of the reaction mixture which can result in a nonuniform distribution of the filler in the finished part.
It is also known from DE 33 13 042 A1 to introduce fillers, e.g. pigments, by means of a bore provided with a throttle device into the reaction mixture. The introduction of the filler is here at the throttle location can give rise to strong pressure fluctuations resulting in damage to pressure-sensitive fillers. Since the introduction is also effected counter to the flow direction of the reaction mixture, this device is also not suitable for the introduction of long fiber fillers.
Further from DE-OS 29 24 554, it is known to mix the filler substances with the reaction mixture in the mixing chamber via an axial bore in which a piston is arranged. The piston is reversibly displaced in a cylindrical chamber from which the reaction mixture is ejected at intervals into the interior of a mold. The displacement of the filler, e.g. of glass fibers, through the axial bore and a lateral supply duct is not simple to accomplish. It can be effected only via a pressurization of the filler in which case it is advantageous to provide the filler in a liquid or doughy state so that it can be forced into the chamber by pumping. With this feed device, however, only short fiber fillers can be fed. Since the glass fibers and possibly the flowable medium transporting the glass fibers are introduced directly into the mixing chamber, there is also the danger that the mixing of the chemically-reactive components will be disturbed and that the required precise stoichiometric mixing ratio for generating a uniform quality of the end products cannot be established.
In the same way there is according to GB 1 245 216 the drawback that a direct introduction of the additive substances into the mixing chamber will interfere with the mixing and give rise to quality defects in the end products.
Furthermore, it is known in the production of plastic parts composed of thermoplastic synthetic resins to feed a melt of a thermoplastic synthetic resin generated in a plastifying unit by means of a movable injection unit via an open mold to thereby obtain a strand of a flowable melt, whereby in the mold ahead or following strand formation, a plaited mat of long reinforcing fibers is laid into the open mold. The mat is then pressed together with a strand-shaped melt to a plastic part reinforced with the elongated reinforcing fibers. This fabrication process is relatively expensive since the plaited mat must be trimmed to the contour of the mold before it is fed into the mold.
It can be necessary, further, to preform the plaited mat in a previous molding process in which case the plaited mat may have to be supplied with a finely divided particulate thermoplastic so that the preformed plaited mat can retain its shape for the next working step.